Composite materials composed of filler materials dispersed in a polymeric matrix are known to exhibit mechanical properties, such as stiffness, strength and toughness, and physical properties, such as coefficient of thermal expansion and electrical and thermal conductivities, which are superior to the polymeric matrix alone. In particular, composite materials formed using carbon fibers dispersed in an epoxy resin have found uses in both the automotive and aerospace industries due to their excellent strength and stiffness per unit weight, as well as their desirable electrical and thermal properties. Generally, optimum mechanical properties for carbon fiber composites are attained with collimated fiber structures within an essentially void-free epoxy matrix. To eliminate the formation of voids, the fibers are typically surface treated to enhance the wettability of the fibers by the epoxy resin.
Carbon fibers are generally produced by heat treating a carbon-containing raw material, such as rayon, acrylic, and polyacrylonitrile (PAN), usually in the form of a polymer fiber. Most commercial products formed from carbon fiber composites utilize carbon fibers derived from PAN. However, the cost of forming these carbon fibers has inhibited the use of carbon fiber composites in many commercial applications.
Recently, manufacturing processes have been developed which are able to produce carbon fibers at significantly lower costs. In particular, U.S. Pat. No. 5,024,818 to Tibbetts et al, assigned to the assignee of this invention, teaches a method and apparatus by which carbon fibers can be catalytically grown by a vapor deposition process from hydrocarbons. The carbon fibers produced by the method taught by Tibbetts et al are generally nanometer-size (i.e., less than about one micrometer in diameter) and therefore significantly smaller than carbon fibers conventionally available, which are generally at least about one micrometer in diameter, and more often on the order of at least about seven micrometers in diameter. In addition, the fibers produced by the method taught by Tibbeus et al are relatively short, with lengths typically on the order of about 40 to about 200 micrometers, and perhaps as small as five micrometers or less. Therefore, such fibers are generally too small to allow the properties of the individual fibers to be measured directly.
Potentially, composite articles formed from nanometer-size carbon fibers would have a microscopically smooth surface, which is highly desirable from an aesthetics standpoint. Furthermore, the potentially low cost of such nanometer-size fibers would make the use of carbon fiber-reinforced composites more economically feasible for a wide variety of applications.
However, there are several significant disadvantages associated with nanometer-size, vapor-grown carbon fibers, such as those taught by Tibbetts et al, which have substantially prevented the production of composite materials made from these fibers. Firstly, nanometer-size fibers are difficult to handle. A mass of nanometer-size carbon fibers has an extremely low apparent density (on the order of less than about 1.times.10.sup.-3 grams per cubic centimeter). Consequently, a quantity of such fibers is difficult to use, and furthermore, a larger volume of these fibers is generally required to obtain a composite composed of a suitably high weight percent of fibers. Secondly, it is very difficult to sufficiently mix nanometer-size fibers with a polymeric material because a high viscosity mixture results when significant amounts of fibers are used, such as on the order of about 5 to about 40 volume percent. Thirdly, the small size of the fibers creates a potential health hazard from inhalation when the fibers are dispersed.
Finally, the vapor-grown carbon fibers are not readily wettable because the basal graphite planes of the fibers are arranged in concentric circles so as to form a low energy surface. Consequently, conventionally-used epoxies will not readily adhere to the fibers, such that the mechanical properties of the composite are reduced by the presence of voids within the matrix. Notably, degradation of tensile strength due to poor adhesion can result in a composite having a lower tensile strength than the epoxy alone, i.e., without the inclusion of carbon fibers. While surface treatments of carbon fibers derived from PAN are often employed to promote wetting and adhesion, such treatments have not succeeded in promoting the wetting of nanometer-size carbon fibers.
Also, as noted above, nanometer-size carbon fibers are too small to allow the properties of the individual fibers to be measured directly. Because prior efforts have failed to successfully produce a composite article with sufficient adhesion between a polymeric matrix and nanometer-size carbon fibers, indirect measurements of the mechanical properties of such fibers have also been essentially prevented. Accordingly, the utility of composite articles reinforced with nanometer-size carbon fibers is not well defined.
Thus, it would be desirable to provide a method by which relatively low cost, nanometer-size, vapor-grown carbon fibers can be utilized to form composite materials, such that composite articles exhibiting suitable mechanical and physical properties can be made, while simultaneously overcoming the above-noted handling and processing difficulties associated with nanometer-size vapor-grown carbon fibers.